How to find the Windows Experience Index in Windows 8.1/8/10 on my PC (computer rating, performance)?

Please open Performance Information and Tools! Use the key combination[Win-Logo]+[R], then simply type the command:control.exe /name Microsoft.PerformanceInformationAndTools ( ... see Image-1 Arrow-1) and confirm that with [ENTER], or press the [OK] button.

Info (c) Microsoft:
The Windows Experience Index is a measurement that tells you how well your PC works with Windows and uses a base score to rate the experience you can expect. A higher base score usually means that your PC will be faster and more responsive than a PC with a lower base score.

The Windows 8 Experience Index base score typically ranges from 1.0 to 9.9. However, if your PC has a 64-bit processor and 4 GB or less random access memory (RAM), then the Memory (RAM) subscore for your PC will have a maximum of 4.9. This means that the overall base score will also be limited to 4.9.

Info (c) Microsoft:
Each piece of hardware in your PC gets its own score called a subscore. The total base score for your PC is determined by the lowest individual subscore, rather than an average of them all. You can view the subscores to see how the hardware you care most about is performing, which can help your decide whether to upgrade some parts of your PC.